1. Technical Field
The presently disclosed embodiments are directed to duplex media registration in a printing system using media edge detection for registration in a first pass and registration marker detection in a second pass.
2. Brief Discussion of Related Art
Accurate duplex printing can be important to reduce or eliminate “show through,” which refers to misregistration or misalignment between images printed on a first and second side of a sheet. For example, when a sheet is held up to a light the images should typically be registered from one side to the other so that the images are aligned. The misregistration between side one and side two printing can also be detectable when the sheets are used in book form and the facing pages are to be aligned (registered). The show through or misregistration of the second side can be a result of variation in paper cut tolerances, registration errors resulting from sensing of that edge and xerographic effects on the paper.
Typically, in duplex printing, media is registered in a first pass to facilitate printing on a first side of the media and is reregistered in a second pass to facilitate printing on a second side of the media. Some printing systems invert or flip the media end-over-end during duplex printing so that the end of the media that was the lead edge for printing on a first side of the media in the first pass becomes the trail edge for printing on a second side of the media in the second pass. Conventional printing systems that invert or flip the media between passes typically detect and register media using the same end of the media. For example, registration based on a lead edge of the media in the first pass, results in registration based on the trail edge in the second pass.
Duplex registration using the same end of the media is typically performed to reduce cut sheet errors. However, there are several issues with these conventional duplex registration schemes. As one example, edge sensors are typically distributed to detect trail edges of different sized sheets. For long sheets of media, such as sheets of about 26 inches in length, it may not be practical to have a trail edge sensor positioned to detect the trail edge. As another example, the trail edge of larger sized sheets of media can be in a curved baffle of the duplex return path while the lead edge can be in position to receive an image, which makes trail edge registration difficult. As another example, the printing process of some printing systems can change the length of the sheet and affect the calculated length from the trail edge to the lead edge. As yet another example, given the wide variety of media lengths being printed, the trail edge of sheets of media for second pass printing in duplex printing may not be sized to be timely and/or accurately detected by discretely and fixedly positioned trail edge sensors.